Exhaust gas recirculation (EGR) is a technique for reducing nitrogen oxide (NOx) emissions from internal combustion engines. In EGR systems, a portion of an engine's exhaust gas is recirculated back to the engine cylinders. The recirculation of the NOx dilutes the O2 in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. In many EGR systems, the recirculated NOx is cooled by an EGR heat exchanger or cooler to allow the introduction of a greater mass of recirculated gas.
With the advent of Tier 4 emission standards, the use of after-treatment systems and other engine modifications like turbochargers, electronic control, EGR systems and the like has become even more common. Most engine manufacturers use high pressure EGR loops to avoid turbocharger fouling and other condensation issues in order to meet the emission requirements. The EGR cooler is one of the most important components of the EGR system, and fouling of the EGR cooler is among the most common reasons for engine failure.
Commonly used EGR coolers are designed for small engines, i.e., <1,000 HP, where exhaust gases flow inside tubes, and water or other cooling fluid is flown over the tubes inside the cooler housing. In contrast, in very large engines such as those used in marine and diesel locomotives, the placement of the fluids is reversed for maximum efficiency. In such implementations, the cooling fluid is flown inside cooling tubes and the exhaust gas is flown unconstrained over the cooling tubes and their fins. It is very common in these types of EGR coolers that the exhaust gas flow enters the EGR cooler through a narrow inlet opening that opens into a larger area of the EGR cooler upstream of an array of cooling tubes. In known EGR coolers, a meaningful portion of the exhaust gas flows around the cooling tube array through a path of least resistance through the EGR cooler housing. The flow of the exhaust gas around the cooling tube array reduces the heat transfer between the exhaust gas and the cooling fluid in the cooling tubes, and correspondingly reduces the heat transfer efficiency of the EGR cooler.